There often occur such accidents as a child being left in a vehicle under the intense heat of summer and dying, because of heat, in the vehicle. To prevent the occurrence of such a tragedy, a system is being studied that detects the presence of a human inside a vehicle, and that issues some kind of alarm when the temperature inside the vehicle becomes excessively high.
FIG. 1 shows one example of a sensor designed to detect the presence of a human, such as an intruder in a vehicle. In the figure, reference numeral 1 is the sensor, and 2 is an object, in this case, a human body such as an intruder. In the sensor 1, an oscillator 11 produces an output, for example, of 2.45 GHz, and this output is radiated from a transmitting antenna 12 toward the human body 2. The reflected wave from the human body 2 is received by a receiving antenna 13, and mixed in a frequency converter 14 with a portion of the transmitted wave to produce a beat signal.
When a moving object is detected inside the vehicle, the sensor determines it as being an intruder. The detection of a moving object is done utilizing the Doppler effect. For example, a radio wave of 2.45 GHz is radiated from the transmitting antenna 12 toward the object 2, and the radiated radio wave is reflected by the human body 2 and received by the receiving antenna 13. If the human body is moving, the reflected wave is slightly shifted in frequency due to the Doppler effect. When the transmitted frequency is fo, for example, the reflected frequency is fo+Δ. Here, the amount of shift, Δ, is derived from the following equation.Δ=Reflected frequency−Transmitted frequency=(2v/c)fo  (1)
v: Relative velocity of human body 2 with respect to sensor
c: Velocity of light
As can be seen from the above equation, the value of Δ is extremely small compared with the frequency of the transmitted wave fo. For example, when the transmitted frequency is 2.45 GHz, Δ is on the order of several tens of Hz. Since it is difficult to measure the amount of shift Δ directly, beats occurring between the transmitted wave and the received wave are measured, and a signal of a frequency equal to the amount of shift Δ is output.
However, if the presence of a child quietly sleeping in a vehicle is to be detected, the only movement that can be detected is the slight movement of the chest associated with the breathing action of the child. In the case of an infant, the chest movement generated by breathing is about 2 mm in amplitude and 0.2 to 0.5 Hz in frequency. If the frequency is 0.5 Hz, for example, the speed of the chest movement is no more than 2 mm per second. Such a minuscule and slow-speed movement is difficult to detect by utilizing the Doppler effect. For example, when the transmitted frequency is 2.45 GHz, the Doppler frequency (the amount of shift Δ) produced by the movement of 2 mm per second is 0.03 Hz according to the above equation. That is, it takes 1/0.03 seconds to obtain a waveform of one cycle. However, since the breathing motion is about 0.5 Hz in frequency, the direction of the motion is reversed before the waveform can be obtained. Therefore, in such a region, the Doppler effect does not occur (Problem 1).
Conventionally, a microwave is used as the transmitted wave, but microwaves have the property that they are reflected by electrically conductive objects but pass through nonconductive objects. As a human body is electrically conductive and therefore reflects microwaves, the advantage is that the chest movement can be observed without being affected by clothes, etc. However, the microwave sensor has the shortcoming that it is susceptible to external disturbances because, as shown in FIG. 2, the transmitted wave passes through the vehicle's windshield which is electrically nonconductive. In FIG. 2, the sensor 1 detects the presence of the human 2 inside the vehicle R, but it also detects through the windshield G the movement of a human P outside the vehicle. On the other hand, as the movement to be detected in the vehicle is a very small breathing motion, the sensor sensitivity must be increased but, in that case, the sensor becomes more sensitive to movements outside the vehicle (Problem 2).
Accordingly, it is an object of the present invention to provide a method for detecting a human body in a vehicle, that can detect a very small movement associated, for example, with the breathing motion of a sleeping child.